In the elevator industry safety is of major significance and a number of developments have arisen over the years. By way of example GB Patent GB 2,414,073 (Airdri Limited) describes a system for gap scanning in a door having a movable leading edge and a fixed location on an opposite side of the door opening between an open configuration wherein a first plurality of emitter units of beams of electromagnetic radiation located on one lateral boundary of a gap and a second plurality of receiver units for the beams is located on the other lateral boundary in which control means provider for the emitter and receiver units to establish an array of beams across the gap in which a beam from one emitter in the first plurality can be received by a complementary receiver in the second plurality; wherein the number of units in the one plurality is of the order of twice the number of units in the other plurality; the plurality containing the greater number of units being located relative to the side of the gap from which the door moves in moving from a position where the variable gap size is a maximum to where the variable gap is a minimum.